1. Field of the Invention
The present invention relates to a vibration damping device for a vehicle body which improves the riding comfort by damping vibrations in the vehicle body.
2. Description of the Related Art
The body of a vehicle elastically deforms slightly when an external force is applied during running of the vehicle, and this elastic deformation generates micro-vibrations of about a few ten μm to 1 to 2 mm. An external force which causes this elastic deformation is applied to the vehicle body from, e.g., a wheel running over a projection or a recess in the road surface or from the engine.
Conventionally, a vibration damping device is sometimes attached to a vehicle body in order to damp a vehicle vibration of this kind. An example of the conventional vibration damping device for a vehicle body is a hydraulic device described in Japanese Patent No. 4627389.
The hydraulic vibration damping device for a vehicle body disclosed in Japanese Patent No. 4627389 includes a hydraulic cylinder unit to generate a damping force, and a linking rod unit connected to a piston rod projecting from the hydraulic cylinder unit. The hydraulic cylinder unit includes a hydraulic cylinder, a piston, and free piston movably fitted in the hydraulic cylinder, and a piston rod which is coupled to the piston.
The piston divides an internal working oil chamber of the hydraulic cylinder into two oil chambers. The piston includes a working oil passage to cause the two oil chambers to communicate with each other, and a damping force generating check valve to open and close the working oil passage. The piston rod projects outside the hydraulic cylinder through one oil chamber.
The free piston partitions the hydraulic cylinder into a gas chamber filled with a high-pressure gas and the working oil chamber.
The hydraulic cylinder portion itself is directly fixed to one end portion of a vehicle body in the lateral or longitudinal direction. On the other hand, the piston is fixed to the other end portion of the vehicle body in the lateral or longitudinal direction via the piston rod and linking rod.
In this conventional vibration damping device for a vehicle body, the piston and hydraulic cylinder move relative to each other due to the vibrations in the vehicle body, and the working oil passes through the check valve of the piston, thus generating a damping force.
A vehicle such as a passenger car or other automobile which runs as it is driven by a driver has been required to further improve the riding comfort. However, the conventional vibration damping device for a vehicle body disclosed in Japanese Patent No. 4627389 does not satisfactorily improve the riding comfort of a vehicle for the following two reasons.
The first reason is that the hydraulic cylinder is directly fixed to one end portion of a vehicle body in the lateral or longitudinal direction as described above, so the length of the transmission path of a vibration to be transmitted to the hydraulic cylinder is different from that of the transmission path of a vibration to be transmitted to the piston. That is, the damping force generated by the hydraulic cylinder unit directly acts on a vehicle body on the hydraulic cylinder side. On the other hand, the damping force acts on a vehicle body on the piston side via a long non-rigid body defined by the piston rod and linking rod. This non-rigid body includes a spring component and elastically deforms. That is, the damping force reduced by the intervention of this non-rigid body acts on the piston-side vehicle body. Therefore, the vibration damping effect in one end portion of the vehicle body is different from that in the other end portion of the vehicle body, and this makes it difficult to further improve the riding comfort of the vehicle.
The second reason is that, if the check valve does not open because the moving velocity of the piston is significantly low, the working oil flows through a so-called leak portion such as a seat portion of the check valve or a sliding portion of the piston. The working oil has a viscous resistance. That is, in this case, a damping force which is large compared to the moving velocity of the piston is generated due to the viscous resistance of the working oil passing through the leak portion. Since this interferes with the relative movement of the piston and hydraulic cylinder, no vibration is damped, and the riding comfort of the vehicle degrades. The case in which the piston moving velocity is significantly low is, e.g., the initial stage of a piston operation or when a vehicle body moderately vibrates to such an extent that a passenger cannot feel it.